Broadcast has an almost century long tradition in radio. Even with TV, the history goes back to 1930's. Broadcasting has been successful throughout the world in bringing both entertainment and information to mass audiences.
The latest step in broadcasting is the digitalization of both radio and TV. Digital radio has not gained much acceptance on the market. However, many hope that digital TV will bring new benefits and services to the consumer and, as a result, generate new revenue streams for the broadcasting industry. The basic concept of the TV service itself has, however, not changed much. Rather, the TV lives on as before even if it has become digital.
In later half of 1990's we saw the boom of the Internet. A whole set of new services and content became available to the consumers during a short, revolutionary and hype intense period. That period introduced e-commerce, Internet Service Providers (ISPs), Portals, eyeballs game, dotcom companies and even the new economy. The developments in both access technologies (e.g. ADSL) and coding technologies (e.g. MPEG-2 streaming) has made it possible to bring rich media content like video content to homes via the Internet. Despite of these technology and market breakthroughs media houses have been reluctant to distribute their content via the Internet due to its “free-of-charge” nature and the direct threat of piracy. Neither has Internet been able to challenge the role of traditional media as the primary advertisement platform despite its great popularity.
Impulsive interference is observed in broadcast to cause difficulties in broadcast reception. This interference may be produced by ignition sparks from vehicles or various household appliances like hair-dryers, vacuum cleaners, drilling machines etc. The cheapest models of these tools often have insufficient interference suppression. Also—for the same reason—single or even burst of pulses occur while switching on or off any device connected to the power line. These could be electrical heating devices, thyristor dimmers, fluorescent lamps, refrigerators etc. This has to be taken into consideration, especially in indoor reception with a simple omnidirectional aerial. Field strength of a broadcast signal, especially for a portable device situated indoors, can be quite low and further weakened by multipath reception. For fixed reception, insufficient cable shielding within in house signal distribution often reduces the benefit of a roof aerial, making the signal reception sensitive to impulsive interference.
One approach in trying to solve the impulsive noise has been based on clipping the impulse bursts. After clipping, the samples are given the value which corresponds to the clipping level amplitude (and keeping the phase). Or the clipped values may be given value zero because these samples are known to be unreliable in any case. An example of the approaches in these lines has been a patent publication EP 1 043 874 A2, incorporated herein as a reference. In this publication, signal levels exceeding certain clipping levels in time domain are detected and those samples are then replaced by zeros. However, this approach leaves the corrupted but unclipped samples untouched which leads to poor signal-to-interference ratio, especially, if the burst power is high. Moreover, the clipping methods leave impulse levels, not detected, untouched which means that their capabilities are limited. Further, the mere blanking of signal makes signal-to-noise ratio poor.
Another known approach in trying to solve the impulsive noise is to blank all the samples that are known to be corrupted, for example, belonging to an interference burst period. The knowledge of impulse position and duration may be based, for example, on monitoring exceeding of certain clipping levels. One such approach is presented in a publication, Sliskovic, M: Signal processing algorithm for OFDM channel with impulse noise. Electronics, Circuits and Systems, 2000. ICECS 2000. The 7th IEEE International Conference on, Volume: 1, 2000, Page(s): 222-225 vol. 1, incorporated herein as a reference. However, this method is too straightforward, since all burst suspected of interference are totally blanked. The modified signal is very different than the original, because all data values within the interference are blank and have no correspondence between the original values. Thus, the mere blanking of signal makes signal-to-noise ratio poor. In order to make the performance of blanking approach better, one could try to solve an equation giving the samples of the original signal that have been removed. If the noise burst is detected and the corresponding time samples blanked, theoretically it might be possible to use the information that there should be no signal on the empty carriers (in the guard band) to restore the original post-FFT values. Such an approach has been described in the referred IEEE publication. Unfortunately the method described in the reference requires a solution of general complex system equations which is cumbersome and heavy (generalized matrix inversion, where dimension of matrix is several hundreds or even over 1000). This is complex and difficult to solve. Also relying only to the spectrum part in the guard band turns out to be inefficient in systems with thousands of carriers received through a noisy channel such as the OFDM system. The missing samples cannot be reliably solved. Moreover, the receiver is unable to perform the required theoretically complex calculation. In addition, information about guard band is too vulnerable to the noise, and solutions are inaccurate. Therefore, an approximate solution for estimate is needed.
Thus, there is a need for a reception which can withstand a higher level of interference such as the impulse interference and improve data reception quality.